The reason for the small value of \(\phi _{\rm tilt}\)  is that the anisotropy energy is much smaller than the exchange interaction energy between the two spins in each domain which tends to keep them aligned antiparallel. For example, the \(ab\) -plane trigonal anisotropy parameter in EuMg\(_2\) Bi\(_2\)  according to Eq. (REF ) below is \(K_3 = 6.5 \times 10^{-8}\)  eV/Eu, whereas the Heisenberg exchange interaction between a spin in one ferromagnetically-aligned layer and the spins aligned in the opposite direction in one of the two nearest layers is \(J_1=1.97\times 10^{-5}\)  eV [1]}.
